"But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and by the Spirit of our God" (1 Corinthians 6:11).
Sanctification is often viewed as the second of three stages in the life of a believer; the first being justification, the second sanctification, and the third glorification.
Sanctification is a process. It begins at conversion and continues throughout the earthly life of the believer. It is the process whereby the believer, empowered by the Holy Spirit, becomes increasingly more Christ-like in character. Paul explains the Holy Spirit's role in sanctification this way: "Now the Lord is the Spirit, and where the Spirit of the Lord is, there is freedom. And we, who with unveiled faces all reflect the Lord's glory, are being transformed into His likeness with ever-increasing glory, which comes from the Lord, who is the Spirit" (2 Corinthians 3:17-18).
Unlike justification, sanctification requires work on the part of the believer. The Holy Spirit does not act alone; we must cooperate with him in the sanctification process: "As obedient children, do not conform to the evil desires you had when you lived in ignorance. But just as He who called you is holy, so be holy in all you do; for it is written: 'Be holy because I am holy'" (1 Peter 1:14-16).
It is important to realize that, while we do play a necessary role in sanctification, we are not left to our own devices for fulfilling our obligation. Not only does God the Holy Spirit work through us, but we have in fact been predestined by God to undergo sanctification: "And we know that in all things God works for the good of those who love Him, who have been called according to His purpose. For those God foreknew He also predestined to be conformed to the likeness of his Son that He might be the firstborn among many brothers" (Romans 8:28-29).
"Therefore, I urge you, brothers, in view of God's mercy, to offer your bodies as living sacrifices, holy and pleasing to God -- this is your spiritual act of worship. Do not conform any longer to the pattern of this world, but be transformed by the renewing of your mind" (Romans 12:1-2).